The flag of the Syrian al-Qaeda-affiliated group Islamic State of Iraq and Greater Syria. Photo: Wikimedia Commons.

A Free Syrian Army source says a San Diego man was killed in battle fighting for the Islamic State (IS) against a rival anti-Assad faction over the weekend. Douglas McArthur McCain, 33, was identified through passport photos and distinctive neck tattoo.

This comes as the Obama administration ramps up its effort to counter the self-proclaimed terrorist state in the wake of American journalist James Foley’s brutal execution last week.

McCain was born in Illinois but later moved to the Twin Cities in Minnesota where he graduated from Robbinsdale Cooper High School in 1999. Police arrested McCain in 2000 on a disorderly conduct charge; in 2001 on a theft charge; and again in 2006 on charges of obstruction, according to court documents.

His “Duale ThaslaveofAllah” Facebook account features jihadist imagery such as the black flag used by IS and links to other Facebook pages depicting street brawls. His most recent Facebook activity shows he befriended a Malaysian jihadist who lives in Raqqa, Syria where the IS is based.

He “reverted” to Islam in 2004, according to a Twitter post. McCain reportedly worked at a restaurant in San Diego and attended Masjid Nur in that city, NBC News reports.

U.S. intelligence officials estimate at least 100 Americans have gone to Syria to fight for a myriad of jihadist factions battling the Assad regime. Of those, only about a dozen Americans are believed to be fighting for IS.

McCain becomes the fourth American known to have been killed while fighting for jihadist groups.

In May, Florida resident Moner Mohammad Abusalha became the first American to be a suicide bomber in Syria fighting for Jabhat al-Nusra, al-Qaida’s Syrian affiliate. A Pittsburgh man, identified as Amir Farouk Ibrahim, was reportedly killed fighting in Syria in 2013. Nicole Lynn Mansfield, a white convert to Islam from the Detroit area, also was killed while fighting there last year.

McCain’s death no doubt will heighten concern about IS ability to recruit American Muslims to fight under its banner – concern that had already been brewing due to images of the White House and Chicago’s Old Republic Building that had been circulated in IS-related Twitter accounts.